During the past two decades, the world has experienced remarkable changes in the telecommunication industry. Communications that were previously carried on the wires are now supplied over radio (wireless). For example, wireless mobile services grew from 11 million subscribers worldwide in 1990 to more than 2 billion in 2005. During the same period, the Internet grew from being a curious tool for academicians and researchers to a virtually mandatory tool for millions of people all around the world desiring to participate in the global community. This staggering growth of the Internet is driving demand for higher-speed Internet-access services, leading to a parallel growth in broadband adoption. In less than a decade, broadband subscription worldwide has grown from virtually zero to over 200 million. The desire to combine the convenience of wireless with the rich performance of broadband has been one of prime driving forces of wireless communication industry.
Digital subscriber line (DSL) technology, which delivers broadband over twisted-pair telephone wires, and cable modem technology, which delivers over coaxial cable TV plant, are the leading mass-market broadband access technologies today. Both of these technologies typically provide up to a few megabits per second of data to each user, while with the newer fiber to the home systems several tens of megabits per second are possible. As Internet applications continue to require more bandwidth, users are demanding ever faster Internet service from the service providers. Fiber to the home systems that can meet the user requirement are, however, extremely costly to deploy, have a long deployment time, and, due to the cost, will generally be exclusive to highly urbanized area. Thus, providing even greater impetus to wireless communication industry to offer users a cheaper, faster, and ubiquitous wireless broadband service.
One of the leaders in the wireless communication industry that are trying to meet the market demand for wireless broadband service are Mobile operators. Mobile operators around the world are upgrading their networks to 3G technology to deliver broadband applications to their subscribers. Mobile operators using GSM (global system for mobile communications) are deploying UMTS (universal mobile telephone system) and HSDPA (high speed downlink packet access) technologies as part of their 3G evolution. Traditional CDMA operators are deploying 1×EV-DO (1x evolution data optimized) as their 3G solution for broadband data. Actual user data rates for these technologies are generally considerably lower than the wired broadband services. For example, using 5 and 10 codes, HSDPA supports peak data rates of 3.6 Mbps and 7.2 Mbps, respectively. However, typical average rates that users obtain are in the range of 250 kbps to 750 kbps. Enhancements, such as spatial processing, and multiuser detection, can provide higher performance over basic HSDPA systems, albeit at a higher cost of deployment. Further, the cell radius of 3G solutions is generally limited to between 1 to 3 miles, which, by virtue of deployment cost, results in limiting the operation of these types of networks to highly urbanized areas.
One of the fundamental challenges for broadband wireless comes from the transmission medium itself. The designs of most current broadband wireless services are such that signals have to travel under challenging Non-Line of Site (NLOS) conditions. Large and small obstructions, terrain undulations, and interference from other signals, together weaken, delay, and distort the transmitted signal in an unpredictable and time-varying fashion. Design of a digital communication system that performs well under these conditions, especially when the service requirements call for very high data rates, is particularly challenging. 3G solutions as well as other NLOS systems all are subject to the limitation imposed by NLOS transmission medium.
Another challenge to broadband wireless comes from the scarcity of radio-spectrum resources. Regulatory bodies around the world have allocated only a limited amount of spectrum for commercial use. The need to accommodate an ever-increasing number of users and offering bandwidth-rich applications using a limited spectrum challenges the system designer to continuously search for solutions that use the spectrum more efficiently. In order to achieve higher system-wide spectral efficiency the concept of a cellular architecture are used. Typically, a small group of cells or sectors form a cluster, and the available frequency spectrum is divided among the cells or sectors in a cluster. The pattern of frequency allocation within a cluster is then repeated throughout the desired service area and is termed frequency reuse. Naturally, reuse of frequency results in increased inference within the same system, which consequently leads to degradation in the data rate transfer. This is particularly true for the systems operating in the NLOS frequencies where the terrain undulation and other structures are not very effective in reducing interfering signals strength.
A wireless broad system designed for accessing the Internet generally requires much higher downlink capacity than uplink capacity. For example, in Web browsing, a predominate Internet application, the uplink data basically contains the URL of the Web site of interest, but the downlink contains the entire content the Web page, which generally includes considerable amount of data intensive graphics and text, among other things. Other applications may have more symmetrical uplink/down characteristics but the average behavior of the link is usually highly asymmetrical. As a result, the selected method of duplexing for the broadband wireless system must be appropriated so as to reflect the asymmetrical nature of Internet data. In frequency division duplexing (FDD), this can be accomplished by allocation of more bandwidth for downlink than uplink, while in time division dupplexing (TDD) allocation of greater number of time slots for downlink verses uplink achieves the desired result.
TDD is highly sensitive to timing and thus not particularly well suited for systems with large cell radiuses. The frequency band allocations for FDD 3G solutions are usually paired so as have similar propagation characteristic. While this feature may prove to be useful for link optimization and employment of single antenna for both transmission and reception, it does not, however, allow for frequency band selection that is advantageously suited to the asymmetrical nature of the Internet data traffic pattern.
To overcome the deficiencies of the present wireless broadband systems U.S. Pat. No. 7,174,127 to Otten discloses a hybrid satellite communications system that includes a satellite system and a terrestrial communications system. As shown in FIG. 1, the satellite system includes two transceivers. The first transceiver receives and transmits a first set of signals received from the terrestrial communications system to a plurality of user units. In reverse fashion, the satellite systems second transceiver receives a second set of signals in a second frequency band from the user units and transmits those signals back to the terrestrial communications system. The first set of signals (downlink signals) are of much higher frequency than the second set of signals (uplink signals) and are relayed by a Direct Broadcast System (DBS) satellite in a LOS frequency band between 12.2 GHz and 12.9 GHz, while the second set of signals are relayed by a Mobile Satellite System (MSS) satellite operating in a frequency between 1.0 GHz and 3.0 GHz, or relayed by a terrestrial node operating between 0.8 and 2.0 GHz.
The use of a 500 MHz band for transmission of downlink signal at first appears to be a significant improvement over the prior art, which are generally limited to tens of MHz. However, this band must be shared with all the potential subscribers that lie in the satellite footprint, which in this includes the entire North America. In addition, this band is also being used for DBS primary service of broadcast television. Thus, the user bandwidth allocation does not appear to have been significantly increased by this method. Even the use of spot beams does not necessarily improve the situation, since the footprint of even a spot beam may include several major metropolitan areas. Further, the significant round trip propagation delay associated with satellites and a terrestrial system can detrimentally influence the user experience for various Internet applications
Accordingly, there is a need for a high speed, low deployment cost, wireless broadband system which provides access to Internet users. It would also be desirable for this system to provide service to urban, suburban, and rural areas and at the same time have low propagation delay.
Moreover, it would be particularly desirable to provide a terrestrial wireless broadband system where the downlink signals are of much higher frequency than the uplink signals and are operating in a LOS frequency band.